1. Field of the Invention
The invention relates to a method and an apparatus for the controlled introduction of a reducing agent into a nitrogen oxide-containing exhaust gas from an internal combustion engine, having a catalytic converter built into the exhaust line for reducing nitrogen oxide.
Due to the pollutant content in the exhaust gas, the use of fossil energy-carriers in internal combustion engines in the traction mode presents major problems in areas with high motor vehicle densities, above all in industrial countries. Pollutants that can be mentioned include, among others, nitrogen oxides, hydrocarbons, carbon monoxide, oxides of sulfur, and soot, which contribute to well-known environmental problems, such as acid rain and smog.
As environmental consciousness increases and as legal requirements in terms of pollutant emissions becomes stricter, a number of catalysts and soot filters have been developed that contribute to reducing the emission of the aforementioned pollutants. In order to reduce pollutants in the exhaust gas of Otto engines, catalysts containing noble metal, for instance, are known in which hydrocarbons and carbon monoxide are converted along with nitrogen oxides and residual oxygen to form carbon dioxide, nitrogen and/or water. In order to reduce pollutant emissions from Diesel engines, so-called particle filters are known which trap the soot particles contained in the exhaust gas. The elimination of the soot particles is carried out by burnoff in the particle filter. Work is also being done in many places to develop a regulated Diesel catalyst, with which the intent is for it to be possible to lower the nitrogen oxide content in the exhaust gas of a Diesel engine considerably. Due to the high residual content of oxygen from the air in the exhaust gas, this cannot be done with the known noble-metal-containing catalysts, of the kind used in vehicles with Otto engines and stoichiometric combustion.
Instead, in vehicles with Diesel and lean-mix engines, that is engines with superstoichiometric combustion, so-called deNOx catalysts are used, with which the nitrogen oxides contained in the exhaust gas are converted with a suitable reducing agent, usually ammonia, into environmentally friendly nitrogen and water by the selective catalytic reduction (SCR) process. The reducing agent or a precursor of the reducing agent is introduced into the exhaust gas upstream of the catalyst in terms of the exhaust gas flow direction and then enters the catalyst in a preferably homogeneous mixture with the nitrogen oxides contained in the exhaust gas.
Internal combustion engines are operated in the traction mode with variable load and rpm. Accordingly, the quantities of nitrogen oxide generated by per unit of time and the flow rates and temperatures of the exhaust gas are subject to major fluctuations. There is currently no known solution for adjusting the quantity of reducing agent introduced into the exhaust gas per unit of time in such a way that high precipitation rates for the nitrogen oxides and simultaneously a vanishingly slight escape of reducing agent are attained regardless of the operating state of the engine. A further difficulty is that ammonia is poisonous, and even at a concentration of only about 5 ppm it represents a considerable odor burden to the human being. For that reason, an escape of ammonia must unconditionally be avoided.
Patent Abstracts of Japan Publication No. JP-A 55093917 teaches the detection of the exhaust gas flow rate, the nitrogen oxide concentration in the exhaust gas, and the exhaust gas temperature, along with the temperature of the denitration catalyst. Through the use of those data and in accordance with the nitrogen oxide rate, a rate of introduction into the exhaust gas is performed that takes into account the catalytic activity of the catalyst, which is dependent on the catalyst temperature. It has been found that such a method, especially at positive and negative sudden load changes, does not prevent an escape of reducing agent or nitrogen oxide.